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LDC-Q4012 — The attached case file cannot be read

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# LDC-Q4012 — The attached case file cannot be read

## What it means

A question over a package can carry its own case: a file with the facts
to reason about. The engine reads that file before reasoning starts. When
the file is missing, unreadable, or not well-formed data, the question
cannot run, and the engine reports the path together with what went wrong
while reading it.

Check that the path is reachable from where the query runs and that the
file holds well-formed data. The question itself stays as written.

## Example

```lawql
query q over "demo.diagnostics" { for x: Applicant; where resident(x); select resident(x); case "/tmp/diag-ref-q4012-no-such-case.json"; }
```

## Fix

```lawql
from r in rules select r
```

## Engine message

The engine reports this in its own wording:

```text
LDC-Q4012: failed to read case /tmp/diag-ref-q4012-no-such-case.json: No such file or directory (os error 2).
```

## Related

- [Questions to the engine](/constructs/queries/) — the shape of a question over a package.
- [LDC-Q4009](/diagnostics/LDC-Q4009/) — when the path itself is not quoted.